In recent years, development of a vehicle that has a function to travel following other vehicle has advanced (for example, Patent Documents 1 to 9 described below). The vehicle automatically travels following the other vehicle (leading vehicle), whereby automatic driving (including semi-automatic driving such as automatic steering) of such a following-side vehicle (following vehicle) can be realized. In this way, a load on a driver of the following vehicle can be reduced, and in addition, this can also contribute to efficient operation of a traffic infrastructure.